In a cellular communication system (like for example the Global System for Mobile communication (GSM) and TETRA (TErrestrial Trunked RAdio)), a geographical region is divided into a number of cells each of which is served by a base station. One base station, also termed as Base Transceiver Station (BTS) with a switching infrastructure is referred as a node of the network. The switching infrastructure supporting BTS may be shared with other BTSs. The system infrastructure in a TETRA system is generally referred to as a switching and management infrastructure (SwMI), which substantially contains all of the communication elements apart from the MSs. This includes base transceiver stations (BTSs) connected to a conventional public-switched telephone network (PSTN) through base station controllers (BSCs) and mobile switching centres (MSCs). In geographical terms reference to a node is a reference to an area served by one BTS, and in network terms reference to a node is a reference to the infrastructure equipment required to support one or more BTSs. A remote unit referred also as subscriber or mobile station (MS) or communication unit is served via a radio communication link by the base station of the cell within which the remote unit is operating.
One of the basic advantages of the cellular communications systems is that subscriber may move from one geographical location to another one while receiving services from the network. To provide seamless service there are regions of overlapping coverage between base stations. As the subscriber moves from area served by a first base station towards area served by a second base station it enters the region of overlapping coverage. Within the region of overlapping coverage the subscriber changes the serving base station. This is known as cell reselection or handover.
The communication link from a BTS to a subscriber is generally referred to as a downlink communication channel. Conversely, the communication link from a subscriber to a BTS is generally referred to as an up-link communication channel.
To enable communication between two handsets operating in two different cells a fixed network interconnects the base stations. The fixed network is operable to route data between any two base stations and this way allows for communication between these two remote handsets. In addition, the fixed network may comprise gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN). This allows subscribers to communicate with landline telephones and other communication terminals connected by a landline. Additionally the fixed network is adapted to perform functions required for managing a conventional cellular communication network including routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
The TETRA communications system may be used as a public cellular communication system. However a principal application of TETRA communication systems is for use by are organizations or groups such as emergency services. Special functions and services implemented in the TETRA system make this system especially suitable for services like police, emergency, fire rescue or others. One of such features provided by TETRA system, which is especially useful for the emergency services is controlling group calls as well as managing the membership of these groups. Other features and services provided by TETRA include, push-to-talk channel allocation, broadcast calls etc. In addition to trunked mode operation wherein remote units communicate via a base station, TETRA provides for the possibility of communication directly between remote units without participation of the infrastructure. This is known as Direct Mode Operation (DMO).
A deployable or ad-hoc network may be considered as one in which the topology of the network and number of nodes which together offer service to mobile users changes from time to time. The connections between these nodes also change in number and topography from time to time, either together with or independently of changes in the nodes. Nodes may be added or may be taken away. Some nodes may offer gateway services, and others may not be able to offer such services. The gateway services available at nodes may also change from time to time, such that at one time a node may be able to offer such services, but at another time, such services may not be available.
In such a deployable or ad-hoc mobile network, where users need to access services outside the network via gateways it can be difficult to determine which gateway should be accessed by a user, and which gateway should receive calls or other information from outside the network to deliver to the user. It can also be difficult to provide a fast set up of call services, especially group services in a truly ad hoc network. When calls are directed to users via gateways, it is expected that calls will be received at a gateway with a connection to the Home Location Register (HLR). This is because the HLR of the user will have information as to from which node the user is currently obtaining service. With an ad hoc network, the connection between any one gateway and the HLR of the user cannot be guaranteed. Calls originating from users may need to route to specific gateways. Identifying the specific gateway can be difficult in an ad hoc network. Gateways may not be available, or may not have external connections available, depending on the state of the network. If mobile users or their HLRs are fully distributed, it is not obvious whether either the node where the mobile user is connected has an available connection, or the node where the HLR is connected has a connection, which makes it difficult to determine how to route a call originated by the mobile user.
A need therefore exists for a method of controlling and identifying location information of subscribers in a communications network, and nodes where gateways are located particularly when the network is required to be operational irrespective of reconfiguration processes, wherein the abovementioned disadvantages may be alleviated or overcome.